1. Field of the Invention
This invention relates to a nuclear magnetic resonance imaging method and, more particularly, to a novel method of applying a high frequency pulse to tip a nuclear spin at a predetermined angle.
2. Discussion of Background
FIG. 4 shows a waveform diagram of parts of signals in a nuclear magnetic resonance imaging method described, for example, on pages 139 to 145 of "Proceedings of the symposium on Nuclear Magnetic Resonance Imaging" magazine issued in 1982. In FIG. 4, symbol Gx designates a first gradient, symbol Gy designates a second gradient, symbol Gz designates a third gradient, symbol SIGNAL designates an NMR (Nuclear Magnetic Resonance, hereinafter referred to as "an NMR signal") signal, and symbol RF designates a high frequency pulse.
In a prior-art nuclear magnetic resonance imaging method, a 90.degree. pulse (RF1) of a high frequency pulse (RF) of a selective excitation pulse is applied to a magnetic field generator, not shown, together with a gradient Gz1 in a third gradient Gz. At this time, the pulses are not applied to the first and second gradients Gx and Gy. Thus, a nuclear spin (not shown) in a thin slab (not shown) having a predetermined thickness in an object (not shown) is selectively excited. Then, gradients Gx2 and Gy2 in the first and second gradients Gx and Gy parallel to the thin slab are applied to the object, and the third gradient Gz is further inverted from the gradients Gz1 to Gz2. Thereafter, after .tau..sub.2 time from the application of the 90.degree. pulse (RF1), a 180.degree. pulse (RF3) of the high frequency pulse (RF) is applied, and the gradient Gx4 of the first gradient Gx is then applied. A spin echo signal is observed in the presence of the gradient Gx4 by the abovementioned operation. Therefore, when the NMR signal is sampled Nx times, if the outputs are Fourier-transformed, the spin density is projected on an x axis. In other words, the second gradient Gy alters the phases of the spins in y direction. A series of the operation is repeated for the second gradient Gy of Ny pieces of the values, and when the outputs obtained from this result are Fourier-transformed, the density value of Nx x Ny array is formed. It is understood that this method can form a two-dimensional image of a predetermined plane in the object.
Since the 90.degree. pulse of the high frequency pulse is used as an exciting pulse in the prior-art nuclear magnetic resonance imaging method, the method has such disadvantages that the influence of the irregularity of the high frequency magnetic field to the NMR signal cannot be ignored and the irregular intensity produced in an image cannot be minimized.